The present invention relates to a method of controlling an optical disk drive or apparatus which irradiates a track of an optical disk with laser light, detects a tracking error on the basis of the reflected light thus produced and automatically controls the gain of a variable gain amplifier of a tracking servo circuit in order to compensate for the detected tracking error.
Generally, the optical disk apparatus for driving a compact disk (CD) or minidisk (MD) includes four servo mechanisms such a tracking servo, focus servo, etc. Among these servo mechanisms, AGC (AGC: Auto Gain Control) is carried out in the servo mechanisms of the tracking, focus and thread.
The AGC, which is one of automatic control functions in a digital signal processor (DSP), intends to control automatically the gain within a filter of the DSP in order to acquire a suitable gain in the servo loop. This compensates for the tracking error or focus error to allow stable reproduction to be made. The AGC is disclosed in Japanese Patent Publication No. 10-269617A, for example.
Meanwhile, in the case of the MD having a small diameter of the optical disks, the tracking servo is problematic. In order to accurately irradiate the track with a laser beam by a pick up so as to track eccentricity occurring in the manufacturing of a disk or chucking, in the tracking servo, a tracking error which is a discrepancy between the irradiated position of the laser beam and the track is detected and an objective lens is moved by a two-axis actuator to control a beam position so that the tracking error is compensated for. Incidentally, the tracking servo is disclosed in Japanese Patent Publication No. 7-153092A, for example.
The method for acquiring the tracking position information or tracking error includes previously known three-beam technique, DPD technique. Among them, the three-beam technique is most common. In the case of the tracking servo by the three beam technique, as shown in FIGS. 4B to 4D, a laser beam is divided into three spots using a diffractive grating so that two subsidiary beam spots A and B are arranged before and after a main beam spot M for signal read. In this case, it is most preferable that the main beam spot M is just above the track T and both subsidiary beams A and B partially overlap the track whereas the remaining part is located on the mirror face of the disk. On the other hand, the states shown in FIGS. 4B and 4D exhibit those where the tracking error has been generated.
The one of both subsidiary beam spots A and B which is located on the mirror face is reflected and the reflected beam is detected by an optical detector. An output from the optical detector is supplied to a circuit as shown in FIG. 4A. In the state of FIG. 4C, since the same quantity of light is detected from both subsidiary beam spots A and B, the output from the circuit shown in FIG. 4A is zero. On the other hand, in the states of FIGS. 4B and 4D, the diffraction of both subsidiary beams spots A and B due to pits is ill-balanced. As a result, in the state of FIG. 4B, the polarity of the output from the circuit of FIG. 4A is positive whereas in the state of FIG. 4D, the polarity of the output from the circuit of FIG. 4A is negative. Thus, the tracking error signals with both polarities can be obtained. Namely, two items of information on which side of one track the beam spot is deviated to and on its quantity of deviation can be obtained.
As is clear from FIG. 5, the curvature radius of track is different in accordance with the radial position of the disk. Namely, the curvature radius of the track at the inner periphery side of the disk is larger than that at the outer periphery side of the disk. Therefore, even if the subsidiary beam spots A and B are adjusted in a proper condition as shown in FIG. 4C on the side of the inner periphery, on the side of the outer periphery, they are shifted unlike that of FIG. 4C.
Further, in the case of the three-beam technique, the output of the tracking signal is influenced by whether the spot position of the laser beam moves accurately perpendicularly to a track tangent line. Therefore, in a case where the AGC is executed at the track licated inner periphery side of the disk, a change in the adjusted gain value is easily produced.
Thus, the AGC in the method of detecting a tracking error by the three-beam technique has a tendency that a gain difference is generated between the inner periphery and outer periphery and a change in the gain adjustment is large on the side of the inner periphery. Therefore, if the AGC for tracking servo is carried out at a fixed position of the side of the inner periphery on the basis of the same rule, the proper gain to be adjusted by AGC will change. This leads to poor tracking in the range of a low frequency and a dropout of sound, otherwise excessive tracking on a flaw in the range of a high frequency. In the case of the MD inner tracks of which situate inner than the inner tracks of CD and CD-ROM, the above problems would be further enhanced.
Conventionally, in many cases, since the AGC for the tracking servo was carried out at the innermost periphery, there is a problem that the gain adjusted by AGC is likely to deviate largely from its proper value.
The problem to be solved by the present invention is to minimize the gain difference by the AGC for the tracking servo between the sides of the inner periphery and outer periphery.
In order to solve the above problem, according to the present invention, there is provided a method of automatically controlling a gain of a variable gain amplifier in a tracking servo circuit incorporated in an optical disk apparatus for compensating a tracking error detected based on reflected light from an optical disk, comprising the steps of:
determining a position where the automatic gain control (AGC) is executed as an optimum position where a gain difference between an inner periphery side and an outer periphery side of the optical disk become small; and
execute the automatic gain control at the determined optimum position.
In such a configuration, the AGC for the variable gain amplifier of the tracking servo circuit is executed at an optimum position where a gain difference between an inner periphery and an outer periphery of the optical disk is smaller. Therefore, the gain difference is minimum so that trackability for eccentricity of the disk and immunity to flaws can be improved. Further, occurrence of dropout of sound is prevented and stabilized tracking can be realized. Such an effect is remarkable in the MD having a smaller diameter than that of CD.
Preferably, the optimum position is determined as a position where a gain A0 of the variable gain amplifier satisfies the following relationships:   A0  =      A1    +                  A2        -        A1            2      
where A1 is a gain obtained by executing the automatic gain control at the innermost track of the optical disk; and A2 is a gain obtained by executing the automatic gain control at the outermost track of the optical disk.
Accordingly, the gain difference between the sides of the inner periphery and outer periphery is minimized.
Preferably, information regarding the optimum position is previously stored in a storage unit. The automatic gain control at the optimum position is executed by reading out the optimum position information from the storage unit.
Accordingly, the AGC for the variable gain amplifier of the tracking servo circuit is executed at the executing position read from the storage unit and hence is executed at a fixed position so that stable tracking can be realized.
Preferably, the optimum position includes a first optimum position where is other than the innermost track and a second optimum position located outer than the first optimum position on the optical disk.
In this configuration, the AGC is not executed at the innermost track providing a largest change in the result of the AGC, but is executed at the other position than the innermost track. Therefore, the change in the result of the AGC can be suppressed to realize a stable tracking.
Preferably, information regarding the optimum position is previously stored in a storage unit. The automatic gain control at the optimum position is executed by reading out the optimum position information from the storage unit.
Preferably, the method further comprising the steps of:
detecting a type of the optical disk; and
reading out the optimum position information in accordance with the detected type of the optical disk.
In this configuration, since the executing position previously introduced according to a kind of the optical disk id stored in a storage unit, the AGC for the tracking servo is executed at an optimum position suited to the optical disk at issue so that stable tracking can be realized.
Preferably, when the detected type of the optical disk is a non-recordable disk, the first optimum position is determined as a position where a gain A01 of the variable gain amplifier satisfies the following relationships:   A01  =      A1    +                  A3        -        A1            2      
where A3 is a gain obtained by executing the automatic gain control at a predetermined reference position on the optical disk; and
wherein the second optimum position is determined as a position where a gain A02 of the variable gain amplifier satisfies the following relationships:   A02  =      A3    +                  A2        -        A3            2      
In this configuration, for the optical disk is a not-recordable disk dedicated to reproduction, the AGC is executed at optimum positions on the sides of the inner periphery and outer periphery at a boundary of the predetermined reference position, and hence stable tracking can be realized.
Preferably, when the detected type of the optical disk is a recordable disk, the first optimum position is determined as a position where a gain A01 of the variable gain amplifier satisfies the following relationships:   A01  =      A4    +                  A5        -        A4            2      
where A4 is a gain obtained by executing the automatic gain control at the innermost track of a pit area on the optical disk; and A5 is a gain obtained by executing the automatic gain control at the outermost track of the pit area; and
wherein the second optimum position is determined as a position where a gain A02 of the variable gain amplifier satisfies the following relationships:   A02  =      A6    +                  A7        -        A6            2      
where A6 is a gain obtained by executing the automatic gain control at the innermost track of a groove area on the optical disk; and A7 is a gain obtained by executing the automatic gain control at the outermost track of the groove area.
In this configuration, for the recordable optical disk dedicated to recording, the AGC is executed at optimum positions within the pit area and groove area and hence stable tracking can be realized.
Preferably, the optical disk is a minidisk.
In this configuration, in the case of a minidisk (MD) having a smaller diameter than CD, since the executing position previously introduced according to a kind of the MD such as a pre-mastered type and recordable type is stored in a storage unit, the AGC for the tracking servo is executed at an optimum position suited to the MD at issue so that stable tracking can be realized and dropout of sound can be prevented.